SMT hand assembly techniques: A question

[Doug S]

I am looking for a better means of holding small SMT components to PCBs prior to soldering. For automated pick n place assembly there are various adhesives for automated dispensing onto the PCB. While I have not actually tried any of these, they don't seem to be suited to hand assembly due to short shelf life and the difficulty of buying in small quantities. The basic problem is getting the component to stay in place so that it is not dragged around by the surface tension of the solder when hand soldering. The method that I currently use is to dilute fingernail polish with acetone to use as an adhesive. I apply with the end of a toothpick. The method is less that satisfactory because I have trouble getting the applied dots small enough to prevent contamination of the solder pads. Also, it does not dry as quickly as I would like and the heat of soldering sometimes breaks the bond. There must be something else that is better and readily available. What have you had success with?

 

[dat2zip]

Doug,

The method I used is the next best method to pick and place machines. I use a stereo microscope and a good SMT tweezers. I can usually do one of the following to "tack" down the part.

1) apply flux with a flux pen to one side or set of pads. Hold the part down tweezers and apply a little solder to the soldering iron and then touch one of the pads. If this works correctly, the solder will wick from the tip and flow onto the component and board simultaneously.

2) Pre-wet one pad. Another technique is to pre-solder one pcb pad. From there, hold the component in place and re-flow the one pad. This should solder or tack that pin down in place. From there, you can reheat that pad and with the tweezers rotate the component as needed for alignment on the board. I use this method for almost all discrete capacitors, resistors, inductors etc...

3) Use SMT solder paste. This technique if done correctly is fast and easy. It takes some eperience, but, it will net you some of the best results. Normally this stuff is stenciled on the PCB with a squegie (?sp) onto the pads using a metal stencil. It's impossible to do the same for hand soldering technique, so don't even try that. Instead, I use alternate technique for SMT ICs and multi-pinned parts. I carefully apply a thin line of SMT past over the pads perpendicular and down the row of pads. For a SO8, I would apply it over pads 1-4 and near the outter portion of the pads. The SMT paste line should be about 1/2 the width of the pads as a start point or guestimation. From there, place you SMT part down. The leads need to land on the SMT paste. Now, hit the leads, either one at a time or multiple pins at a time with the iron. This will liquify the SMT paste, flux the pins, and solder them all in one shot. Carefully, inspect for loose SMT paste balls, and re-heat as needed to suck up any remaining loose solder paste balls. I use this method for some of the fine pitched ICs.

BTW: Pick N Place machines use the SMT paste to hold the part down before reflow unless parts are going to be placed on both sides of the board, in which case the first pass, all the parts have an epoxy dot added so that when they flip the board over to place the 2nd side, the 1st side parts don't fall off. Otherwise, for a single component side it's only paste, then place.

I'm about 1/2 as fast as a good SMT hand assembler. What takes the shop 5 minutes to solder, will take me 10 minutes. That's my average over many boards.

Good luck with your soldering.

Wayne

 

[Jonathan]

I am a big proponent of method 2) above for two terminal components like resistors

I use really fine wire solder. I touch the soldering iron to one PCB pad, feed in solder, and make a little mound of solder on the pad. Then I hold the component with tweezers in one hand, the soldering iron in the other hand, melt the solder mound, and slide the component in. Then I solder the other side of the component.

I also second the recommendation for using solder paste for things like surface mount ICs and the like.

-Jon

 

[Doug S]

Wayne, thanks for the detailed answer. You raise another question for me. So far, I have avoided buying a stereo microscope. Maybe I should revisit the idea of getting one considering that my eyesight has deteriorated quite a bit in recent years. For assembly work, what features should I be looking for, e.g., total magnification, etc? Now if you can just suggest a cure for shaky hands...
Anyone with stereo microscope knowledge, please jump in too.

 

[PsycoBob[Q2]]

I don't know about stereo scopes, but I was tempted to get a cheap $40 'Discovery-channel' USB microscope. 5/10/20x, and I could use it just like any USB camera. With webcam drivers I could do lots of things in software- rotate, mirror, color-balancing, glare-reduction, specific color-inversion, magnification on one point with a normal view on the other part of my screen...

I wish I'd grabbed one while they were local, and on sale for Christmas.

 

[dat2zip]

A basic one would be a fixed base, variable magnification 0.7X - 3X head, 10X optics. That would give you 7X - 30X. 90% of my work is done at the 7X level.

Surprisingly, your shakey hands don't get magnified. Your hand eye coordination remains about the same.

You can get by with a Optivisor. problem with the optivisor is you might burn your nose while soldering. The more magification, the closer the field of focus.

You can buy a reasonable one on ebay. Be aware, they are in high demand, and they typically get overbid to well past the 50% list. I finally bought one that had a buy now button and bought it for 50% list. No auctioning needed.

Of course the best part of my setup is not the stereo microscope or the Metcal soldering station. It's my two Mccans I use for lighting the work surface.

Each McCan has a 1W Luxeon, an early proto of the Wizard board, loc-line and a plastic box base. I have one on either side to generate cross shadows. Works great.

If you get more than 10X power, it will be too strong for SMT work. The Intel microscope (now defunct I think) and others are more for standard biology experiments.

You probably can even get by with a fixed 7X.

If you have bad eyes like me, regular magifiers, lighted magnifiers don't cut it.

You need stereo magnification, good lighting, and loads of patience.

For tiny SMT work, you will need the fine solder wire as well.

Expect to pay $900 - $1500 For a boom mounted Baush and Lomb stereo microscope.

For many knockoffs, (oversees imports), expect to pay $200.00 - $300.00. There's no cheatin here. Save your eyes... Great for taking out slivers too. Dual purpose.

(Easier to sell to your other half...)

There is one other option. A good surgeon optical glasses. The ones that look like monoculars attached to glasses. Yup, very funcky looking. But, they magnify and have a focal distance of arms length. Man, I would like one of them. I don't even want to look to see how much they would cost...

Come to think of it, I'm sure the cost would be saved over time in ease of use, back strain, and being able to use them anywhere like during delicate assembly steps, soldering and other many tasks. It's amazing how often my stereo scope gets used. I can imagine have a good set of glasses would get used even more.

 

[Lucien]

I too make use of Wayne's method (2) above. However, so far, the smallest stuff I've done is 0805 case size resistors.

I've seen a nice guide (with pics) of method (3). It's here at the Seattle Robotics Society. At least I think that's what Wayne meant. /ubbthreads/images/graemlins/smile.gif They also make use of a toaster oven to solder some stuff.

[ QUOTE ]
dat2zip said:
If you have bad eyes like me, regular magifiers, lighted magnifiers don't cut it.


[/ QUOTE ]

Wayne,

Are those magnifiers worth it at all? I've been thinking of getting one; Haven't really seen any microscopes where I get most of my tools from, and the "wearable" stuff available seems a bit questionable

 

[dat2zip]

http://www.zeiss.de/C125679B00315836?Open

A starting point for what may be the ultimate stereo vision with magnification. I have no idea how much these cost or if you need special prescriptions or what. I need to contact my optometrist and see if they can help me.

Lucien,

It depends on what your age is. If you are under 30, you may not need anything at all. I've seen others just use a plain simple magnifyer held in one hand, and soldering iron in the other (though not very efficient).

It's kinda up to you to determine what you need and what you can afford to buy. Not everyone can afford a Metcal soldering station and a stereo microscope.

The optivisors are pretty good for starters. Check them out carefully and remember the focal distance. Don't get one that is so strong the focal distance is 3" since you will tend to burn parts of your face with the soldering iron. (Been there, done that...) /ubbthreads/images/graemlins/smile.gif

 

[INRETECH]

When I worked at Intel, one of my jobs was filling out purchase orders - the technician came to me and said that the manager gave him permission to order a microscope, he brought me the two that he liked - one for $1500 and one for $7000; I didn't even ask him which one I should order

Two weeks later it came, 52lbs - I had to have a shop-person drill 3x10mm holes in the desk to mount it

It was SOO nice !!

 

[Jonathan]

I tried the toaster oven soldering technique, and it worked reasonably well, but I decided that I like using the soldering iron better. For small parts (like the 10 pin ICs with the 0.5mm lead spacing), I prefer the solder paste, but use it with the soldering iron.

-Jon

 

[paulr]

Wayne, please check PM? I need advice about something off-topic. Thanks.

 

[Mednanu]

Re: SMT hand assembly techniques: A question *DELETED*

Post deleted by Mednanu

 

[dat2zip]

OK,

Well, I made the jump. I found some of the glass frame versions on ebay. Like anything else, they have a retail price of $900.00 or more.

They have several magnifications powers, and I was hoping to get the 3.5X, but, the ones being auctioned are full framed versions. Having prescription glasses I thought the clip on ones would be more generic. We'll see.

The ones I bought are here:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=2929924667&category=29954&rd=1

Wayne

Don't look if you get queasy over a $100.00 item...

 

[McGizmo]

Well I got a pair of loupes as well and I have used them on a mod I am currently trying to keep from becoming labor intensive scrap. Without these loupes, I think I would have already failed at this, my most ambitious undertaking to date. It's so dang nice to actually see what you're working on! I think next on the list is a highpowered head lamp.

- Don

 

[Doug S]

Man, you guys are cruel! I came here hoping to find a marginal improvement over my $2 a bottle fingernail polish adhesive technique and now I am lusting over equipment that retails for over $1000.
Thanks to all for the input.

 

[INRETECH]

When I worked at Intel, I was setting up a new dept and needed a microscope - the manager said buy the technician anything he wanted, he came to me with datasheets on a $1200 microscope and a $7000 microscope

I didn't ask him which one he prefered

Two weeks later, I got a 52lb box in my office - had to get the building man to come out and drill 3 large holes in one of the desks for this monster !

The light was SO intense that you couldn't look directly at the PCB where you were soldering, the image thru the viewer was a LARGE 4x6 viewer that I could easily put my entire face into including my glasses...